1. Field of the Invention
The present invention relates to an apparatus for controlling the amount of slip of a lock-up clutch provided in a power transmitting system of a motor vehicle
2. Discussion of the Related Art
In a motor vehicle having a fluid-filled power transmitting device equipped with a lock-up clutch, such as a torque converter or fluid coupling incorporating such a lock-up clutch, it is proposed to control the lock-up clutch in a slip control mode (partially slipping mode) such that an actual amount of slip (slip speed) of the lock-up clutch, namely, a difference between the speeds of a pump impeller and a turbine impeller eventually coincides with a predetermined target slip speed, for the purpose of improving the fuel economy of the vehicle while minimizing the power loss due to slipping of the lock-up clutch. The slip control mode is established when the running condition of the vehicle is in a predetermined slip control area which is intermediate between a fully releasing area in which the lock-up clutch should be held in a fully released state, and a fully engaging area in which the lock-up clutch should be held in a fully engaged state. These fully releasing, fully engaging and slip control areas are defined by suitable parameters (e.g., throttle valve opening and vehicle running speed) indicative of the vehicle running condition.
Generally, a lock-up clutch whose slip speed or amount is adjustable is provided with a piston which is operated by a hydraulic pressure source that permits full engagement of the lock-up clutch. Described in detail, the piston is moved depending upon a difference between hydraulic pressures in two oil chambers, which are formed on the opposite sides of the piston. The amount of slip of the lock-up clutch is controlled by controlling the pressure difference of the two oil chambers to thereby change a thrust force acting on the piston and the resulting friction force of the clutch. Since the hydraulic pressure source that permits the full engagement of the lock-up clutch is utilized to control the clutch in the slip control mode, even a small amount of change in the pressure difference of the two oil chambers will result in a considerable amount of change in the slip amount of the lock-up clutch. That is, the slip amount of the clutch controlled in a feedback fashion tends to be excessively sensitive to a change in a slip control signal or output generated by a feedback control system. In view of this drawback, the slip control output for controlling the amount of slip of the lock-up clutch is according to a control equation which includes a feed-forward control value, a feedback control value and a learning control value.
A slip control apparatus as described above includes slip control means for controlling the amount of slip of the lock-up clutch in the slip control mode, diagnosing means for detecting a defect, abnormality or failure associated with the slip control of the lock-up clutch by the slip control means, and slip control terminating means for commanding the slip control means to terminate the slip control of the clutch when any defect associated with the slip control is detected by the diagnosing means. For instance, it is proposed to terminate the slip control of the lock-up clutch and place the clutch in the fully released state if the slip speed of a torque converter (difference between the input shaft speed and the output shaft speed of the torque converter) is held outside a permissible or normal range for more than a predetermined length of time. An example of such slip control apparatus is disclosed in JP-B-62-7430.
In the conventional slip control apparatus described above, however, the slip control of the lock-up clutch is not terminated if the slip speed of the torque converter changes at random or vibrates along a waviness curve such that the slip speed instantaneously exceeds the predetermined threshold value. In such situation, too, the slip control of the lock-up clutch is considered defective, even though the time period during which the slip speed of the torque converter is kept outside the normal range does not exceed the predetermined length of time. In other words, the diagnosing means does not permit highly reliable detection of a defect, abnormality or failure associated with the slip control.